UM171 targets chromatin-bound MYC for degradation in a CRL3^KBTBD4 dependent manner. (A) Western blot analysis of total proteins extracted from OCI-AML1 cells exposed to DMSO, UM171 (250 nM), or BRD4 inhibitor ARV771 (1 μM) in presence or absence of the neddylation inhibitor MLN4924 (1 μM) for 4 hours. Representative blots showing BRD4, RCOR1, MYC, and KBTBD4 (loading control) protein levels. (B) Western blot analysis of total proteins extracted from day 3–expanded CD34⁺ CB cells exposed to DMSO, UM171, DMSO/MLN4924, or UM171/MLN4924 for 4 hours. Representative blot showing RCOR1, MYC, BRD4, and CAND1 (loading control) protein levels. (C) Western blot analysis of total proteins extracted from sgAAVS1- or sgKBTBD4-engineered OCI-AML1 cells exposed to DMSO or UM171 (250 nM) for 4 hours, with or without MLN4924 (1 μM). Representative blots showing RCOR1, LSD1, HDAC2, MYC, and CAND1 (loading control) protein levels. (D) Western blot analysis of cytoplasmic vs chromatin bound proteins extracted from OCI-AML1 exposed to DMSO or UM171 (250 nM) for 1 hour with or without MLN4924 (1 μM). Representative blots showing LSD1, MYC, and CAND1 (loading control) protein levels. (E) Western blot analysis of cytoplasmic vs chromatin-bound proteins extracted from day 3–expanded CD34⁺ CB cells exposed to DMSO, UM171, DMSO/MLN4924, or UM171/MLN4924 for 4 hours. Representative blots showing MYC, RCOR1, TUBA, and NCL (loading controls) protein levels. See also supplemental Figure 4 for quantitative analysis of MYC protein levels.